The invention relates to a centrifuge comprising a rotatingly mounted bowl and, concentrically rotatingly mounted therein, a scroll rotating at a differential speed which are powered by a central exterior (stationary located) motor assembly and interposed therebetween—where provided as the gearbox controllably defining said differential speed—a hydraulic motor with its casing, on the one hand and its rotor, on the other, the feed of which is provided by a hydraulic pump.
Centrifuges of the aforementioned kind are such in which a gearbox is interposed between two parts rotating at different speeds, the two parts being termed bowl and scroll, i.e. as in decanting centrifuges as mainly intended herein. Involved in particular are solid bowl scroll centrifuges, also screen bowl and full pool decanters. Likewise involved are the number and nature of the phases of the infed solids/liquid mixture, also termed suspension, separated from each other by the centrifugal force. These centrifuges can be provided for two-phase solids/liquid separation, but also for multi-phase separation involving multiple liquid or solids phases. Separation occurs because of the difference in the specific weight of the phases to be separated by centrifugal force. Centrifuges of this kind belong to prior art in a broad spectrum.
It is basically possible to power the two parts of the centrifuge revolving at different speeds by separate stationary motor assemblies (drive motor plus gearbox). But it is regularly the case that only one stationary drive motor assembly is provided for powering one of the two parts, preferably the bowl, from the powered motion of which the rotary motion of the other part is derived by a gearbox defining the differential speed between the two parts. In this arrangement it is important that the torque to be communicated between the two parts is particularly high as a rule. The interposed gearbox needs to be suitable for handling these high torques. Hitherto for these interposed gearboxes, for example, high performance epicycloidal gearboxes were provided which work however with a fixed translation ratio, with the result that one could not take into account differential speed changes that would be needed for many regulation requirements and for adaptions to the corresponding suspension to be processed. Another possibility of achieving such an interposed gearbox is a hydraulic motor which is simple to control because of the simplicity in dimensioning the flow of the hydraulic fluid as regards its speed. Such a controlled variable difference in speed is particularly of advantage, because depending on the particular suspension to be processed separation can be optimized simply by trial and error. In this respect it needs to be taken into account that the flow of suspension per unit of time and particularly its consistency is not uniform, requiring a combination of open and closed loop control. This may result in, for example, an added risk of the machine becoming plugged by accumulations in the solids phase materializing from the suspension parameters not being constant, etc. Such accumulation are indicated by an increase in the torque of the scroll, whereupon one can, by increasing the differential speed, achieve an increase in the solids outfeed in thus counteracting the risk of plugging. Also particularly problematic is that when the central exterior motor drive assembly is down, the stationary bowl can no longer be freed of the solids having sedimented there. Examples of such a centrifuge whose interposed gearbox is achieved by a variable speed, high torque, slow-running, corotating hydraulic motor read from the patents FR 542 659, FR 69 42 189 and U.S. Pat. No. 3,923,241. Necessary for feeding such hydraulic motors interposed between bowl and scroll is a pump assembly arranged exterior and stationary as evident from prior art as cited above, because the flow of hydraulic fluid feeding the hydraulic motor needs to be communicated from a stationary feed circuit into the rotating centrifuge system. This is achieved—for instance, analogously to an electric motor by slip ring/brush—by means of a so-called rotary feedthrough. Because of the substantial hydraulic fluid feed flow at high pressure deriving from a high displacement volume the rotary feedthrough is subject to high demands both as regards flow and as regards pressure so that the size and particularly also the leakage problems involved in this problematic component make for major difficulties. Thus, although such a corotating hydraulic motor combines salient advantages as to lightweight design, good closed loop control or regulation and rugged operation, the rotary feedthrough needed hitherto is a drawback.